Thursday, May 28, 2009

WBIR

What in the heck is that? WBIR stands for Wide Band Image Rejection. The Flex system is a 4th method phasing system, so it looses the images by mathematically canceling the image. Filter rigs rely on the filter to choke out the image and to the extent the filter can act as a choke is the extent you get canceling. If the filter is a crummy choke you get crummy canceling and there isn't anything you can do about it. We generally don't think much about this because the filters do a fair job and if you can't do anything about it why worry? On the other hand the Flex radio can do something about it. An equation can be tuned to within an inch of its life. It is not a piece of physical hardware like a filter. It is plastic and can be shaped.

The above picture (a) represents a signal split into 2 components, component A and component B. In (b) component B is phase shifted 90 degrees from A. In (c) the two components are vectorily added together. The result in picture is A2 cancels B2, and you are left with A1 and B1 adding together. If A2 and B2 are perfect opposites their sum equals zero. If A1 and B1 are perfectly the same you wind up with a signal that is twice the value of either one alone. This is how a phasing radio works. In the computer it is possible to as Picard on Star Trek used to say "make it so". It is possible to make A2 and B2 the same but opposite in sign so you get very deep canceling.

Here are a couple of slides from Gerals's Dayton presentation this year that shows what this means practically.

In the above picture you see some quite strong signals on the left side of the screen. On the right side you see their images.

In this picture we see the same strong signals on the left, but NO signals on the right. The difference is WBIR. When WBIR was engaged it analyzed the signals and essentially made A2=-B2 and the images perfectly canceled.

In the early SDR-1000 code you had to do this adjustment by hand. You would feed in a signal generator signal and then proceed to adjust phase and gain control to cancel the image. On a single freq you could get quite deed nulls, over 100dBm, but as you moved off frequency that null would deteriorate. You might set the null at 3750 for example and be 100dBm down, and move to the bottom of the band and have that deteriorate by many tens of dB. The next move was to remember the image numbers on a per band basis. Each band would have its own null. The system was automated. Code was developed so the radio would null itself on each band, but still only at one point, so there was a variability in image rejection across the band. This was an improvement.

Now we come with what is essentially just in time image rejection across the band. N4HY along with KE5DTO have once again merged their enormous brains and have come up with an algorithm that adapts the image rejection to near perfect on the fly. The algorithm is a very very fast adapting system that nulls the radio to "best null" as you tune to a frequency. If you tune to a new freq the algorithm kicks on and once again you are living in the land of best null. If you are on the same freq for a while, the radio occasionally checks the null, so you are always living in the land of best null. This algorithm is now working in both the main RX and the second RX, and is also going to address TX. This means the radio will always be "best tuned" and perfectly aligned for both TX and RX, no matter where you are in the spectrum including if you are using the radio to drive VHF/UHF/SHF transverters. You will be assured of the cleanest signals both on TX and RX.

Just sit back and think about how cool this is. In the old days you needed a room full of equipment to align your radio. Now all you need is an equation!!

Call to arms

Hear Ye Hear Ye

If you're interested in writing something interesting for this blog regarding your SDR experience let me know. I would like to include things like how your SDR contest station is set up, or your VHF station that uses SDR as the system center, feats of weak signal work or how well the SDR works in various challenging situations. If you are a foreign ham and would like to comment on the growth of SDR in your particular part of the world. Bring it on!

I reserve the right to publish or not, but I'm pretty open to documenting a wide variety of honest experiences from users, for readers to explore. The understanding of SDR in our hobby is so nascent, that I want the reader to be able to see the value of SDR through the eyes of YOUR experience and enthusiasm.